This invention relates generally to condition responsive sensors having a full bridge circuit such as a bridge circuit comprising piezoresistive elements and more particularly to apparatus and methods for providing fault detection which enables detection of errors in either the offset or sensitivity of a sensor that can be smaller than a full scale output signal.
Bridge circuit sensors are widely used to sense a change in a certain condition, such as a change in pressure or a change in acceleration. A common bridge circuit sensor comprises piezoresistive elements mounted on a substrate, such as silicon, so that changes in the sensed condition causes a change in stress induced in the several piezoresistive elements to provide an output of the bridge which is a function of the change in the sensed condition. Typical uses include acceleration sensors for use in automotive braking systems and pressure sensors for use in automotive fuel injection systems, by way of example. There is a continuing need, in many applications, to minimize costs associated with the sensors while at the same time providing detectivity of sensor flaws which can result in an erroneous in-range sensor output. It is known to use redundant sensors to add to the reliability of a system; however, this adds to the cost of the system. It is desired to provide a single sensor system in which the user is alerted in the event of a malfunction of sensor operation.
In PCT application, International publication number WO99/01777, dated Jan. 14, 1999, a circuit is disclosed for monitoring the function of a full wheatstone piezoresistive sensor bridge circuit. If the compared signals deviate by more than an acceptable amount the output of a comparator provides an alarm signal. The disclosed circuit monitors to some extent the bridge resistors as well as the connectivity to an ASIC. However, the cited prior art is complex and is relatively expensive due to its implementation size. The approach in the cited prior art requires two separate conditioning circuits which must be precisely adjusted and stable over time and temperature.
An object of the present invention is the provision of a method and circuit for detecting faults in a bridge circuit sensor simpler and more cost effective than the above noted prior art. Another object of the invention is the provision of an in-range fault protection system for providing an alarm whenever a fault occurs in a full bridge sensor. Yet another object of the invention is the provision of an in-range fault protection system for a full wheatstone piezoresistive bridge type sensor suitable for fabricating in the form of an integrated circuit which is reliable yet low in cost. Yet another object of the invention is the provision of such a system which can be used for other full bridge arrangements, such as a bridge containing capacitive elements.
Briefly described, the fault detection system made in accordance with the invention comprises circuitry connected to the outputs of an energized full wheatstone bridge to provide a means to generate a common mode voltage of the bridge outputs during sensor calibration which is then compared to threshold voltages to determine a fault status during normal operation. The common mode voltage of the full bridge outputs is defined as the voltage equal to a linear combination of the bridge output voltages which is insensitive to stimulus applied to the full bridge and is within the voltage range between the two bridge outputs. Circuit parameters that define the common mode voltage and fault comparator thresholds are appropriately adjusted and stored in nonvolatile memory.